1. Field of the Invention
This invention relates to polycarbonates and more particularly to polycarbonates having improved hydrolytic stability.
2. Description of the Prior Art
Polycarbonates derived from reactions involving organic dihydroxy compounds and carbonic acid derivatives have found extensive commercial application because of their excellent mechanical and physical properties. These thermoplastic polymers are particularly suited for the manufacture of molded products where impact strength, rigidity, toughness, thermal and dimensional stability as well as excellent electrical properties are required.
Further in many applications, especially those requiring extensive service in the presence of heat and light, polycarbonates are used because of their resistance to these constant conditions. The polycarbonates resistance to extensive light and heat is in many instances attributable to stabilizers which prevent deterioration of optical and molecular properties. Among the most extensively used stabilizers are the neutral esters of phosphorous acid including the alkyl and aryl phosphites and the oxetane phosphites. More extensive discussions involving some phosphite stabilizers can be found in U.S. Pat. No. 3,794,629. Although these stabilizers are effective in protecting the polycarbonate from heat and light they cause the polycarbonate to deteriorate, due to hydrolysis, when exposed to water under elevated temperatures.
In some applications polycarbonates are provided with enhanced flame retardant characteristics by using aromatic diols with bromine or chlorine atoms which have a tendency to free radically dissociate and degrade the polycarbonate. To prevent this degradation phosphites have also been added. However, the phosphites, just as in the case of the non-halogenated polycarbonates have a tendency to make the polycarbonate more susceptible to hydrolytic attack.
Thus in accordance with the present invention polycarbonates are provided having improved hydrolytic stability over conventional stabilizers yet with the other desirable properties of the polycarbonate left substantially unmodified.